SUMMARY

Sound-and-orientation recording tags (DTAGs) were used to study 10 beaked
whales of two poorly known species, Ziphius cavirostris (Zc) and
Mesoplodon densirostris (Md). Acoustic behaviour in the deep foraging
dives performed by both species (Zc: 28 dives by seven individuals; Md: 16
dives by three individuals) shows that they hunt by echolocation in deep water
between 222 and 1885 m, attempting to capture about 30 prey/dive. This food
source is so deep that the average foraging dives were deeper (Zc: 1070 m; Md:
835 m) and longer (Zc: 58 min; Md: 47 min) than reported for any other
air-breathing species. A series of shallower dives, containing no indications
of foraging, followed most deep foraging dives. The average interval between
deep foraging dives was 63 min for Zc and 92 min for Md. This long an interval
may be required for beaked whales to recover from an oxygen debt accrued in
the deep foraging dives, which last about twice the estimated aerobic dive
limit. Recent reports of gas emboli in beaked whales stranded during naval
sonar exercises have led to the hypothesis that their deep-diving may make
them especially vulnerable to decompression. Using current models of
breath-hold diving, we infer that their natural diving behaviour is
inconsistent with known problems of acute nitrogen supersaturation and
embolism. If the assumptions of these models are correct for beaked whales,
then possible decompression problems are more likely to result from an
abnormal behavioural response to sonar.

Introduction

Marine mammals have remarkable abilities for breath-hold diving. The best
known deep divers include the elephant seals, Mirounga angustirostris
and M. leonina, which can dive for up to 2 h to depths over 1500 m
(DeLong and Stewart, 1991;
Hindell et al., 1991;
Le Boeuf et al., 1988), and
the sperm whale, Physeter macrocephalus, which can dive for >1 h
to depths in excess of 1185 m or more
(Watkins et al., 1993). Marine
mammals that dive so deep usually do so to forage on mesopelagic or benthic
prey, but elephant seals are also thought to dive to avoid near-surface
predators such as killer whales and sharks
(Le Boeuf and Crocker,
1996).

Deep-diving, air-breathing animals face a number of challenges relating to
prolonged breath-holding and high hydrostatic pressure
(Kooyman, 1989). Animals that
prolong apnea must optimize the size and use of their oxygen stores, and must
deal with the accumulation of lactic acid if they rely upon anaerobic
metabolism. As hydrostatic pressure compresses gases in the body, there are
direct mechanical effects, physical effects such as bubble formation during
decompression and also possible physiological effects such as nitrogen
narcosis, high pressure nervous syndrome, and shallow water blackout
(Bennett, 1982;
Bennett and Rostain, 2003).
Pathologies related to effects of pressure are well known among human divers
(Brubakk and Neuman, 2003), but
marine mammals appear to have developed adaptations to avoid most mechanical
and physiological effects (Kooyman,
1989). The hazard of bubble formation during decompression is best
known for humans breathing compressed gases, but empirical studies
(Paulev, 1965;
Paulev, 1967;
Ridgway and Howard, 1979) and
theoretical considerations (Houser et al.,
2001; Scholander,
1940) have shown that breath-hold divers can develop
supersaturation and possible decompression-related problems when they return
to the surface. Supersaturation has not been measured during normal diving
behaviour of wild marine mammals but rather in specially designed experiments
performed by trained subjects. Nitrogen tensions in the muscles of two
dolphins (Tursiops truncatus) trained to perform a series of dives to
100 m were estimated to be about three times the ambient pressure
(Ridgway and Howard, 1979), a
level of supersaturation that did not appear to harm the subjects. By
contrast, Scholander (Scholander,
1940) reported abundant gas bubbles in the arteries of a
bladder-nosed seal (Cystophora cristata) that died after being
forcibly submerged to 300 m in 3 min and drawn to the surface in 9 min. It is
not currently known what levels of supersaturation lead to pathological
conditions in marine mammals nor if dangerous levels of supersaturation occur
in wild animals conducting voluntary dives and, if so, under what
conditions.

Air-breathing animals rely upon oxygen for aerobic metabolism; anaerobic
metabolism is less efficient and causes the build-up of metabolites, such as
lactic acid, that must be processed aerobically. Deep-diving marine mammals
extend their dives by an increased ability to store oxygen compared to
terrestrial mammals, by increasing their tolerance to hypoxia and by
decreasing their metabolism during dives
(Butler and Jones, 1997;
Kooyman and Ponganis, 1998;
Reed et al., 1994;
Sparling and Fedak, 2004).
Many deep-diving species are able to perform most dives using aerobic
metabolism, but must rely increasingly on anaerobic pathways during long dives
that cannot be entirely supported aerobically
(Butler, 2001;
Kooyman, 1985). The duration
of apnea at which lactic acid starts to accumulate significantly is called the
aerobic dive limit (ADL) (Kooyman et al.,
1980) or diving lactate threshold
(Butler and Jones, 1997). The
measured ADL for Weddell seals, one of the best-studied species, is about 21
min for an adult (Kooyman et al.,
1980), but varies as a function of activity level
(Davis and Kanatous, 1999).
Studies by Kooyman and coworkers (Kooyman
et al., 1980) have shown that almost all (>90%) of the dives
that the seals undertake are within the ADL. When Weddell seals dive longer
than the ADL, up to more than 70 min
(Kooyman, 1981), they build up
lactic acid and usually do not make another long dive until they have
metabolized all or some of it during rest at the surface or during shorter
recovery dives (Castellini et al.,
1988; Castellini et al.,
1992; Kooyman et al.,
1980). This increased recovery time required after anaerobic dives
reduces the overall fraction of time that can be spent under water compared to
a series of aerobic dives (Butler and
Jones, 1997; Kooyman and
Ponganis, 1998). Models of optimal deep foraging suggest that the
cost of dives beyond the ADL would exceed their benefit except in cases where
prey are so deep that they cannot be reached otherwise, or the quality of the
current prey patch is so high as to outweigh the extra time required to
prepare for the next dive (Mori,
1998; Mori, 1999).
Although the details are not fully understood, it is likely that the
management of gas under pressure and anaerobic metabolism constrains the
diving behaviour of marine mammals on a dive-by-dive basis
(Butler and Jones, 1997;
Kooyman and Ponganis,
1998).

Beaked whales are considered to be deep divers based primarily upon diet
inferred from stomach contents (Blanco and
Raga, 2000; Mead,
1989; Podesta and Meotti,
1991; Ross, 1984;
Santos et al., 2001). This
family includes some of the world's most cryptic and difficult to study
mammals and little is known about their diving behaviour. Tagging studies have
revealed that the northern bottlenose whale, Hyperoodon ampullatus,
is capable of diving for >1 h to depths in excess of 1500 m
(Hooker and Baird, 1999).
Recently, Johnson et al. (Johnson et al.,
2004) used acoustic recording tags (DTAGs) to describe
echolocation clicks produced by Cuvier's and Blainville's beaked whales
(respectively Ziphius cavirostris and Mesoplodon
densirostris) during dives of up to 1270 m depth. Both species were found
to produce trains of regular clicks with occasional fast sequences, called
buzzes, during deep dives (Johnson et al.,
2004). Echoes from targets in the water were recorded by tags on
both species, often just before and during buzzes
(Madsen et al., 2005), which
along with concomitant impact sounds and movements are highly suggestive that
buzzes indicate attempts to capture prey
(Johnson et al., 2004). It was
concluded that both beaked whale species forage by echolocation in deep
water.

The diving physiology of Cuvier's and Blainville's beaked whales is of
special interest because of reports of the presence of gas and fat emboli in
these species during atypical mass strandings associated with the use of naval
sonar (Fernandez et al., 2005;
Jepson et al., 2003). The
atypical mass strandings may involve more than 10 animals distributed over
tens of kilometres of coastline within a few hours of sonar transmissions
(Cox et al., 2006;
D'Amico, 1998;
Evans and England, 2001;
Frantzis, 1998;
Martín et al., 2004;
Simmonds and Lopez-Jurado,
1991). Other known causes of stranding have been ruled out in some
cases, and sonar sounds spread rapidly enough over broad enough ranges to be a
potential trigger for strandings with the observed timing and distribution
(D'Amico, 1998;
Evans and England, 2001).
Jepson et al. (Jepson et al.,
2003; Jepson et al.,
2005) and Fernandez et al.
(Fernandez et al., 2005)
report that some stranded cetaceans show indications of gas and fat emboli.
They suggest that exposure to sonar sounds may cause a decompression-like
syndrome in deep-diving whales either by changing their normal diving
behaviour or by a direct acoustic effect that triggers bubble growth
(Houser et al., 2001). The
latter scenario would, however, only seem to happen for animals with 100-223%
supersaturated tissues within tens of meters from a sonar where the received
levels exceed 210 dB re 1 μPa (Crum and
Mao, 1996). Houser et al.
(Houser et al., 2001) suggest
that beaked whales might have levels of supersaturation as high as 300%, for
which bubble growth could occur at lower acoustic exposures than those
reported by Crum and Mao (Crum and Mao,
1996). Nonetheless, the geographical pattern of strandings
suggests that animals are impacted at ranges significantly greater than those
required for acoustically driven bubble growth
(Cox et al., 2006), implying
that the observed pathologies may follow from a behavioural response that has
adverse physiological consequences.

A major limiting factor in determining the probability of in vivo
bubble formation and risk of decompression is the lack of information on the
normal diving patterns of the impacted species of beaked whales. Here we
provide the first detailed quantification of the diving behaviour of Cuvier's
and Blainville's beaked whales using data obtained from multi-sensor DTAGs on
10 individuals. We show that these animals forage on a food source so deep
that the whales on average dive deeper and longer than reported for any
air-breathing animal that dives to forage. We discuss the physiological
implications of this extreme diving behaviour in the light of current models
of diving physiology and recent theories on decompression sickness and
sonar-induced mass strandings.

Materials and methods

Field sites and visual observations

Cuvier's beaked whales (Ziphius cavirostris Cuvier, hereafter
referred to as Ziphius and Zc) were tagged in June of 2003 and 2004
in the Ligurian Sea, Italy. Blainville's beaked whales (Mesoplodon
densirostris Blainville, hereafter referred to as Mesoplodon and
Md) were tagged in October 2003 and 2004 off the island of El Hierro in the
Canary Islands. Both field sites are in deep (700-2000 m) water with steep
bathymetry. In Liguria, 20 m (2003) and 15 m (2004) motor yachts were used as
observation vessels whereas a 10 m motor yacht (2003) and a 7 m rigid-hulled
inflatable boat (RHIB, 2004) were used in El Hierro. Both vessels towed a
small RHIB for tagging. Sighting and tagging of beaked whales usually required
sea states of two or less. In El Hierro, a shore observation station with an
altitude of about 100 m was used to help locate the whales.

The tag

Beaked whales were tagged with a non-invasive sound and orientation
recording tag with overall dimensions 20×10×3 cm (DTAG)
(Johnson and Tyack, 2003).
Acoustic data were recorded from one or two hydrophones in the nose of the tag
sampled at either 96 or 192 kHz using 16-bit resolution sigma-delta
analogue-to-digital converters. A pressure sensor and three-axis
accelerometers and magnetometers were sampled at 50 Hz to measure the
orientation and depth of the tagged whale. The pressure sensor for each tag
was calibrated over a 0-2000 m water depth range before and after the
experiments in a pressure test facility at WHOI. The orientation sensors were
verified after each deployment by measuring their values for a sequence of
defined orientations. Data were stored digitally in 3.3 or 6.6 Gb of
non-volatile memory and a custom loss-less audio compression algorithm was
used to extend the recording time. Tags were attached to whales with a set of
four 6 cm diameter silicone suction cups using a handheld pole. Tags were
programmed to release from the animal by venting the suction cups at the end
of the recording time if they were still attached. A VHF beacon in the tag
aided tracking and recovery of the device. Once recovered, the tag data were
off-loaded to a computer, checked for errors and archived. Each tag deployment
was assigned a code comprising the species initials, the year (two digits),
the Julian day, and the tag deployment of the day (a single letter), e.g.
Md03_298a.

Sound data analysis

The sound recording from each tag was evaluated with a custom visualization
tool in MATLAB. Apart from a few isolated clicks, vocalizations only occurred
during deep dives and comprised long sequences of regular clicks interspersed
with occasional buzzes and short pauses
(Johnson et al., 2004;
Madsen et al., 2005;
Zimmer et al., 2005). The time
of each buzz, and the start and end of regular clicking were noted for each
dive. Buzzes have a lower apparent level in the tag recording than regular
clicks (Madsen et al., 2005)
and can be difficult to detect in Ziphius if the tag is placed low on
the side of the whale or when the ambient noise is unusually high
(Aguilar de Soto et al., 2006).
In two recordings (Zc03_260a and Zc04_161b) buzzes were not reliably audible
whereas in another recording (Zc04_161a) buzzes were not clearly audible after
the first dive because of a change in location of the tag on the whale.

Orientation and movement analysis

Signals from the non-acoustic sensors were adjusted according to
calibration constants and decimated to a sampling rate of 5 Hz. Orientation
signals were corrected for the position of the tag on the whale and then
transformed to estimate pitch, roll and heading of the whale using the method
described in Johnson and Tyack (Johnson
and Tyack, 2003). Changes in the tag position were located in the
data by comparing the tag-derived roll and heading at the surface with the
actual heading and behavioural observations of the tagged whale made by visual
observers. If the tag moved during a dive and the precise time of the move
could not be determined from the data, orientation measurements from that dive
were excluded from further analysis.

The duration of each dive was timed from when the whale left the surface to
when it regained the surface as revealed by the calibrated pressure sensor.
The tagged whales swam to depths of a few body lengths in-between respirations
at the surface, although logging within a few meters of the surface was also
occasionally observed. To avoid counting short shallow submersions in-between
individual respirations as dives, only those instances when the whales went
deeper than 20 m were classified as a dive. Intervals containing dives to less
than 20 m and logging at the surface were scored as surface time. Fluking
motion appears in the sensor recordings as cyclic variations in the
accelerometer and magnetometer signals. A fluke stroke was counted whenever
there was a cyclic variation in the pitch of the whale [i.e. rotation around
the transverse axis of the whale (Johnson
and Tyack, 2003)] with peak-to-peak magnitude greater than 3°
in a time period between 0.3 and 4 s, broadly centred around the nominal
fluking period of between 2 and 2.5 s for Mesoplodon and
Ziphius. The fluke count was verified by inspection of random
sections of the pitch data. Although the tag does not contain a speed sensor,
if it is assumed that the whale swims in the direction of its body axis, swim
speed, s(t), can be estimated by:
(1)

where dp(t)/dt is the rate of change in depth at
time t and ρ(t) is the pitch angle at time t.
This is only a useful estimate when the absolute pitch angle is not small,
i.e. when the whale is ascending or descending steeply. Where swim speed is
reported, this is the average of s(t) over the interval.
Where vertical velocity is reported, this is computed as the depth change over
the interval divided by the duration.

Statistical testing

Owing to the difficulty in tagging beaked whales, the data set reported
here is small both in terms of number of individuals and numbers of dives per
individual. Nonetheless, the rarity and utility of these data motivate us to
offer some basic statistical inferences about diving behaviour. Comparisons of
parameters across different phases of each dive (e.g. descent time
versus ascent time) for each individual were made using paired
two-tailed t-tests. When pooling paired parameter data from
individuals of the same species, log transforms were used as necessary to
ensure homoscedasticity. A significance level of 0.05 was used throughout for
hypothesis testing and the estimated probability, P, is reported as
0.0 if P<0.001. For several analyses, we performed the same
statistical test on several parameters from each individual whale or each
dive. The goal of these analyses was to explore the robustness of the pattern
for each specific test, not to test the same inferences repeatedly for each
parameter across individuals or dives, so within these series of tests, no
Bonferroni corrections were made.

Statistical comparisons between the two species were avoided because of the
low and unequal sample sizes, and variable number of dives per individual. For
correlations, individual means were subtracted from observations which were
then log-transformed and pooled within each species. Individuals for which
there were fewer than three observations were excluded from correlations. One
tag (Md04_287a) was programmed to record audio and sensor data for 9 h, after
which sensor recording continued to a reserved section of memory. The data
section without audio was included in the dive analyses that did not require
breakdown into vocal and non-vocal phases.

Results

Tags were attached to seven Ziphius and three Mesoplodon
over four field efforts in 2003 and 2004, as summarized in
Table 1. Two additional short
(<15 min) attachments to Ziphius were excluded from the table and
analyses. All Ziphius were tagged in the Ligurian Sea and all the
Mesoplodon were tagged in the Canary Islands. Attachment durations
averaged 8.6 h (Zc) and 12.2 h (Md) with five tags remaining attached to their
full memory capacity. Representative dive profiles for each species are given
in Fig. 1 with the timing of
regular clicking and buzzes superimposed. Each panel in
Fig. 1 covers two dives below
500 m extracted from longer recordings. All dives deeper than 500 m were found
to contain long sequences of echolocation clicks, and we define these as deep
foraging dives (Johnson et al.,
2004; Madsen et al.,
2005). In comparison, dives shallower than 500 m were apparently
silent, excluding a few isolated sounds. The separation of these two classes
of dives is apparent in the scatter plots shown in the left column of
Fig. 2 in which dive duration
is plotted against maximum depth for all dives recorded from Ziphius
(top) and Mesoplodon (bottom). A gap in dive depths between 450 and
700 m coincides with the break point between silent shallow and deep vocal
dives. In line with the likely different functions of the shallow and deep
dives, we present results for each dive class separately, following an
evaluation of the effects of tagging.

Summary dive statistics for all tagged individuals rejecting shallow
dives prior to the first deep dive

Tagging effects

Beaked whales were observed to respond to tagging by changing heading,
diving and swimming rapidly. These responses appeared to be both mild and
short-term, but shallow dives immediately following tagging may well be
affected. As a precautionary step, we exclude shallow dives and surface
intervals preceding the first deep dive from analyses. In order to evaluate
whether the first deep dive after tagging was affected by tagging, we compared
the duration and depth of the first two deep dives performed by each whale.
Two Ziphius were excluded from the analysis: one performed only one
deep dive while tagged and the other waited 73 min after tagging before
performing a deep dive. For the remaining eight whales, neither dive duration
nor maximum depth varied significantly between the first and second deep dive
(paired t-test, P=0.8 for duration and P=0.4 for
depth) although the power to detect an effect is quite low, given the small
sample size. Based on this result, we included all deep dives in subsequent
analyses.

Diving data set

Table 1 lists descriptive
statistics for the data set in terms of individuals whereas
Table 2 provides the statistics
of dives pooled across all tagged whales within the same species. Given the
wide spread in number of deep dives recorded from each animal (1 to 9), no
attempt was made to correct the values in
Table 2 for individual
variation or for number of replicates from each individual. Instead, two
standard deviations are reported for each parameter: the standard deviation of
the pooled observations (listed as `s.d. total' in
Table 2) and the standard
deviation of the sample mean of the parameter for each individual (listed as
`s.d. means'). If the two measures of variation differ, then one cannot assume
that the statistics pooled across dives are a good predictor for other
individuals.

Descriptive statistics of deep and shallow dives, pooling dives from
all individuals of the same species

Deep foraging dives

Deep dives were recorded from all individuals tagged for more than 15 min,
with the depth and duration ranges noted in
Table 1. Maximum dive depths
and durations of 1888 m and 85 min were recorded for Ziphius and 1251
m and 57 min for the smaller Mesoplodon
(Table 2). We associate deep
dives with foraging because of the presence of regular echolocation clicks and
buzzes in these dives along with echoes from what appear to be prey
(Johnson et al., 2004;
Madsen et al., 2005). Although
some clicks may also serve a social function, the frequent presence of echoes
from items in the water ensonified by the clicks
(Johnson et al., 2004)
suggests that prey location is a central function of these clicks. We
therefore interpret the time during which the animal produced echolocation
clicks in a dive as time spent primarily searching for prey [paralleling the
usage described by Watwood et al. (Watwood
et al., 2006), for sperm whales]. The duration of this vocal phase
is shown as a function of dive depth for both species in the left-hand panels
of Fig. 2 (unfilled triangles).
An average of 33 min (Zc) and 26 min (Md) were devoted to the vocal phase in
each dive (Table 2), amounting
to an average of 56% (Zc, s.d. 8%) and 53% (Md, s.d. 5%) of the dive duration.
In dives with audible buzzes, the average number of buzzes per dive was 30
(s.d. 12) for Zc and 29 (s.d. 9) for Md
(Table 2). Although buzzes were
produced in a broad depth range from 222 to 1885 m, 95% of all buzzes occurred
between 613-1297 m (Zc) and 463-1196 m (Md), showing that foraging was
concentrated in a broad layer from mesopelagic to bathypelagic depths.

The duration of deep dives is not correlated with maximum depth in
Ziphius but is significantly correlated in Mesoplodon (Zc:
23 dives/4 individuals, r=-0.2, P=0.3; Md: 14 dives/2
individuals, r=0.72, P=0.004). Vocal time, a proxy for time
spent searching for prey, is correlated with maximum dive depth for both
species (Zc: 23 dives/4 individuals, r=-0.47, P=0.02; Md: 9
dives/2 individuals, r=0.81, P=0.008), but is negatively
correlated in Ziphius and positively correlated in
Mesoplodon. It appears that Ziphius must reduce search time
as dive depth increases whereas a Mesoplodon can prolong deeper dives
to accommodate an increased search time, although this determination is based
on few individuals. The range of dive depths recorded from Ziphius is
twice that from Mesoplodon even though 95% of buzzes in both species
occur in a similar depth range.

For deep dives, we considered the descent to extend from the surface until
the whale began to produce regular echolocation clicks. Likewise, the ascent
was considered to start at the last regular click and end at the surface.
Descents were always performed faster than ascents (paired t-test on
vertical speed Zc: t27=16.5, P=0.0; Md:
t10=22.6, P=0.0). Overall, the mean vertical
descent speed was remarkably constant at 1.5 m s-1 (s.d. 0.11) for
Zc and 1.6 m s-1 (0.21) for Md and was uncorrelated with dive depth
(Zc: 23 dives/4 individuals r=-0.1, P=0.7; Md: 9 dives/2
individuals r=0.2, P=0.7). Descents were steep, with mean
pitch angles ranging between 60° to 83°. Fluke rate was high at the
start of descent, dropping substantially for all Zc and one Md (Md03_284a)
within the first 50 m of descent (fluke rate in the 20-40 m depth bin was
greater than in 60-80 m for 27 of 28 Zc descents and all 5 Md03_284a descents)
as exemplified by the fluke rate profile in
Fig. 3. This reduction in fluke
rate was not accompanied by a change in estimated swim speed (paired
t-test 20-40 m depth bin vs 60-80 m, Zc: P=0.4,
Md03_284a: P=0.1). The other two Md maintained their initial fluke
rate throughout the first 100 m of descent but this was accompanied by a
significant increase in swim speed for Md04_287a (same test,
t6=-5.8, P=0.001).

Unlike the descents, ascents from deep foraging dives were always performed
with low overall vertical speed [0.7 m s-1 (Zc: s.d. 0.2, Md: s.d.
0.1); see right-hand panels of Fig.
2] and low pitch angle. Because of the low pitch angle, swim speed
cannot be estimated accurately for ascents and our analyses are restricted to
vertical speed. The overall vertical speed for ascending Mesoplodon
was positively correlated with dive depth (9 dives/2 individuals
r=0.8, P=0.005) and the increased ascent speed on deeper
dives stabilized overall ascent time: ascent duration was uncorrelated with
dive depth for this species. For Ziphius, neither ascent duration nor
vertical speed were correlated with dive depth, and the ascent strategy of
this species is uncertain. However, if the ascents are divided into depth
bins, some patterns emerge. In particular, both species showed a strong
correlation between dive depth and vertical speed in the deepest 200 m of the
ascent immediately after the end of clicking (Zc: 23 dives/4 individuals,
r=0.78, P=0.0; Md: 9 dives/2 individuals, r=0.87,
P=0.002), i.e. both species began their ascent faster from deeper
dives. In comparison, there is little or no correlation between ascent speed
and dive depth in the top 200 m of the ascent (Zc: 23 dives/4 individuals,
r=-0.1, P=0.6; Md: 14 dives/2 individuals, r=0.50,
P=0.07). (All 14 dives from 2 Md were analysed for final ascent speed
whereas only the 9 dives from the same individuals with audio recordings could
be analysed for initial ascent speed as the start of ascent is determined by
the end of clicking.)

Representative sections of dive profiles from Ziphius cavirostris
(A) and Mesoplodon densirostris (B). Intervals with regular click
vocalizations are indicated by a thicker trace and times of buzzes are
indicated by small open circles. The depth of occurrence of buzzes heard in
the audio recording during the two dives is shown in the histogram on the
right-hand side of each panel. The bin size of the histogram was 50 m.

Scatter plots of dive duration (A,C) and vertical speed (B,D) as functions
of dive depth for all dives deeper than 20 m recorded on Ziphius
cavirostris (A,B) and Mesoplodon densirostris (C,D). The plots
in A and C show the surface-to-surface dive duration (dots) and the interval
from the start to the end of regular clicking (unfilled triangles) in each
dive. The absence of dive depths between 450 and 700 m for both species and
the observation that only dives deeper than this range have consistent
vocalizations leads us to define these as deep foraging dives (DFD). The plots
in B and D show vertical speed (i.e. depth rate) as a function of dive depth
during descents (downwards pointing black triangle) and ascents (upwards
pointing unfilled triangle). The difference between descent and ascent rate
for DFDs is apparent.

For Mesoplodon, the fast initial ascent on deeper dives serves to
compensate the longer ascent distance. For Ziphius, the picture is
more complicated. Regardless of the maximum dive depth, Ziphius
ascended slowly between 600 and 400 m from the surface and then ascended more
rapidly during the last 200 m of ascent (paired t-test 0-200 m
vs 400-600 m depth bins: t27=5.2,
P=0.0). For this species, the mean vertical speed in the last 200 m
of ascent was twice that at 400-600 m and the vertical speed in the top 100 m
of ascent did not significantly differ from that on descent (paired
t-test Zc: t27=-1.5, P=0.1). Thus
Ziphius tend to ascend rapidly in the first few hundred meters from
deeper dives then slow down as they pass 600-400 m and finally speed up again
near the surface. This distinctive pattern occurred in 24 of 27 Zc ascents by
seven individuals.

The average pitch angle in the descent of deep foraging dives was steeper
than in the ascent for all dives of both species (paired t-test Zc:
t26=18, P=0.0; Md: t10=10,
P=0.0). Whereas descents were performed with a uniformly steep pitch,
the pitch angle was low and variable during ascents. For both species, ascent
pitch angle and vertical ascent speed were persistently correlated
[correlations performed separately for each ascent divided into 100 m depth
bins: 24 of 27 (Zc) and 12 of 14 (Md) ascents had significant correlations]
whereas fluke rate and vertical ascent speed were not [same test: 10 of 27
(Zc) and 2 of 14 (Md) ascents had significant correlations]. Changes in ascent
speed are therefore achieved primarily by modulating the pitch angle rather
than by changing swimming effort. In fact, as exemplified by
Fig. 3, the fluke rate is
fairly constant during ascent from deep foraging dives. Over all ascents, the
average fluke rate was 10.8 flukes min-1 (s.d. 2.3) for
Ziphius and 19.6 flukes min-1 (s.d. 3.0) for
Mesoplodon which, when compared to the typical steady fluking period
of 2.3 s (Zc) and 2.1 s (Md), implies that whales were actively swimming
during an average of 41% (Zc) and 69% (Md) of each ascent with the remainder
being spent in gliding. Gliding and fluking periods were intermixed with a
typical burst of 1-3 fluke strokes followed by a glide of similar or shorter
length. The fluke rate in the final 40 m of the ascent [Zc: 7.9 per min (s.d.
8.4) and Md: 6.9 min (s.d. 6.3)], although variable, was much lower than
during descents [Zc: 31 min-1 (s.d. 0.9) and Md: 33
min-1 (s.d. 6.9)], (paired t-test, Zc:
t26=-15, P=0.0; Md: t13=-15,
P=0.0). This suggests that the final meters of the ascent are powered
by the buoyancy force of air expanding in the lungs more than by active
swimming.

To test whether whales tended to swim in a steady horizontal direction
during the prolonged ascents, we computed the horizontal distance travelled
during ascent assuming a constant swim speed (sensu
Johnson and Tyack, 2003) and
compared this to the distance that would have been covered at the same speed
if the heading had been constant throughout the ascent. By using a ratio of
distances like this we avoid the need to estimate swim speed which cannot be
done reliably for ascents with low pitch angles. Both species adopted fairly
constant headings during ascent with 24 of 27 Zc dives and all 11 Md dives
covering more than 50% of the possible horizontal distance and 13 of 27 (Zc)
and 8 of 11 (Md) covering more than 80%. The number of buzzes recorded during
the bottom phase and the proportion of horizontal distance covered during the
following ascent (on 11 Zc and 9 Md dives) were not significantly correlated
for either species although this determination is based on just two
individuals from each species with clearly audible buzzes in three or more
dives.

Dive profile (A) and fluke rate (B) for a Ziphius. Vocal phase and
buzzes are marked on the dive profile. The average fluke rate in 1 min
intervals, measured by counting cyclic variations in the pitch signal from the
tag, is shown in the lower panel. The fluke rate is high in the first minute
of both deep and shallow dives but then drops markedly during the remainder of
the descent. Ascents, in comparison, feature more steady fluking.

Foraging dive cycle

We define the inter-deep-dive-interval (IDDI) as the interval between the
end of one deep dive and the start of the next deep dive. The IDDI varies
widely for both species (Table
2) with mean values of 63 min (Zc) and 92 (Md) min. The duration
of deep dives is positively correlated with the preceding IDDI for
Ziphius (Zc: 17 dives/3 individuals, r=0.6, P=0.01;
Md: P=0.08) whereas it is weakly correlated with the following IDDI
for Mesoplodon (Md: 14 dives/2 individuals, r=0.6,
P=0.03; Zc: P=0.14), indicating at least a trend of
increased IDDI in compensation for longer bracketing foraging dives.

Combining the IDDIs with the deep dive durations, the average foraging dive
cycle (i.e. from the start of one deep dive to the start of the next) lasts
121 min (s.d. 36, range 52-222) for Ziphius and 139 min (s.d. 51,
range 62-236) for Mesoplodon. These figures indicate that both
species would perform about 11-12 deep dives per day if diving continues
throughout the diurnal cycle at the pace recorded by the tags. In terms of
diurnal coverage, tags were on whales from 13.00-07.00 h for Ziphius
and 09.20-03.40 h for Mesoplodon, and so no data are available for
the late morning for Zc or early morning for Md.

Shallow dives

A distinctive feature of the dive profiles for both species is the series
of shallow dives performed between deep dives. The IDDIs contained a median of
2 (Zc) and 6 (Md) shallow dives with 17 of 22 Zc IDDIs and all 15 Md IDDIs
containing at least one shallow dive. The left side of
Fig. 2 plots the depth and
duration of shallow and deep dives for both species. The duration of shallow
dives is strongly correlated with their maximum depth (Zc: 65 dives/7
individuals, r=0.8, P=0.0; Md: 100 dives/3 individuals,
r=0.6, P=0.0). Although the maximum depths of shallow dives,
425 m (Zc) and 240 m (Md), overlap with the depth range of buzzes recorded in
deep dives, there is no evidence of echolocation-mediated foraging in shallow
dives. In fact, only three (Zc) and one (Md) buzzes, amounting to about 0.5%
of the total, occur during deep dives at depths attained during shallow
dives.

For shallow dives, the descent period is defined as the time from leaving
the surface until the maximum depth of the dive was reached, and the ascent
period is the time from the maximum depth to the surface. This change of
definition as compared to deep dives is necessary because of the absence of
regular clicks in shallow dives. For both species, the descent period in
shallow dives is, on average, 53% (Zc, s.d. 12%; Md, s.d. 14%) of the total
dive duration. Vertical speeds in descent and ascent do not differ
significantly (Fig. 2). Similar
to the finding for deep dives, the fluking rate is higher during descent than
ascent through the 0-20 m depth range (paired t-test, Zc:
t64=14, P=0.0; Md: t99=7,
P=0.0). However, after the initial burst of fluking in the descent,
the trend is reversed with the ascent fluking rate significantly higher than
descent (paired t-test, Zc: 100-120 m, t54=-6,
P=0.0; Md: 20-40 m, t63=-6, P=0.0),
although the depth at which this changeover takes place differs for the two
species. The lowest fluke rates during the entire tag records occurred during
the later part of shallow descents.

Aligned dive profiles during inter-deep-dive-intervals (IDDIs) for
Ziphius cavirostris (A) and Mesoplodon densirostris (B). The
preceding deep dive ends at minute 0 whereas the following deep dive (not
shown) starts after 10-142 min (Ziphius) or 25-181 min
(Mesoplodon). The decreasing trend in the depth of shallow dives with
time elapsed since the preceding deep dive is apparent.

The dive profiles during each IDDI, starting at the end of the preceding
deep dive, are shown in Fig. 4
for each species. It is evident that shallow dives immediately following the
deep dive tend to be deeper and longer than those occurring later in the IDDI.
To characterize this dependence in a way that is robust to individual
variations in shallow dive sequences, we made a paired comparison of the mean
dive depths in the intervals 0 to N and N to 2 N
minutes after each deep dive, where N was chosen as the mean shallow
dive duration for each species. The mean dive depths in the two intervals for
each IDDI were log-transformed for variance homogeneity and then pooled for
all animals of the same species. The results using N=15 and 10 min,
respectively, for Zc [mean shallow dive duration of 15.2 (s.d. 5.2) min] and
Md [mean shallow dive duration of 9.3 (s.d. 2.4) min], indicate a significant
difference (paired t-test, Zc: t18=3.1,
P=0.006; Md: t12=2.8, P=0.015) in mean
dive depth in the two intervals. An even stronger result was obtained from a
paired comparison of mean dive depth in the N min just prior to a
deep dive and the preceding N-min interval (paired t-test,
Zc: t18=-3.7, P=0.002; Md:
t12=-7.6, P=0.0), confirming that the trend of
decreasing dive depth with time elapsed since a deep dive continues throughout
the interval between deep dives.

Several IDDIs contained protracted surface intervals (i.e. continuous
periods at depths <20 m). Three Ziphius performed a total of seven
surface intervals longer than 30 min (max. 86 min) whereas only one similarly
protracted surface interval was observed in Mesoplodon (max. 33 min).
The mean depth during these surface intervals varied between 1.6 m for a Zc
that logged persistently for 79 min and 10.8 m for the Md that performed a
series of very shallow dives.

Discussion

We report data from a limited set of 10 animals of two difficult to study
species, and discuss the data in the light of physiological implications for
diving physiology and possible decompression problems. The observed diving
behaviour in all the tagged animals shows the same pattern, and is similar to
that reported by Baird et al. (Baird et
al., 2006) for the same species in Hawaiian waters, but we
acknowledge that the data set is limited and that the conclusions we reach
here should be treated with reservations inherent to a limited sample size.
The dives recorded here of up to 1888 m and 85 min are among the deepest and
longest recorded for any air-breathing animals. While there are indications of
deeper dives in sperm
whales†, this
is the deepest dive confirmed by a precise tagging record. Sperm whales
typically perform dives of 30-50 min duration to depths between 600 and 1200 m
(Watwood et al., 2006;
Whitehead, 2003). Hooker and
Baird (Hooker and Baird, 1999)
tagged Hyperoodon and reported dives to 1530 m and for up to 70 min.
Hobson and Martin (Hobson and Martin,
1996) reported a 153 min dive from a Berardius arnuxii
surfacing in a breathing hole in the ice, but they considered that they might
have missed a surfacing, so discarded dives lasting longer than 70 min.
Elephant seals have been reported to dive to 1500 m for as long as 120 min
(Hindell et al., 1991;
Stewart and Delong, 1990).
However, such extreme diving behaviour is unusual for elephant seals that
normally undertake foraging dives lasting about 25-30 min to depths of about
500 m (Hindell et al., 1991;
Le Boeuf et al., 1986;
Le Boeuf et al., 1988). The
important point with the diving behaviour of the beaked whales studied here
is, likewise, not what they can maximally achieve, but the fact that they
perform the deepest and longest average foraging dives reported to date of any
air-breathing animal.

Judging by the vocal and movement data recorded from the tagged beaked
whales, deep dives are primarily used for foraging
(Johnson et al., 2004;
Madsen et al., 2005).
Echolocation tends to start at 400-500 m depth while the whales are still
descending, presumably to find the patch or depth layer richest in prey, and
continues through a vocally active bottom phase. Buzzes, which are associated
with feeding, are centred on the broad depth range of 500-1300 m for both
species. Echolocation behaviour and frequent prey echoes recorded by the DTAGs
(Johnson et al., 2004;
Madsen et al., 2005) show that
beaked whales hunt individual, probably small, prey items of which they try to
catch about 30 per dive. This foraging strategy takes time as each item is
selected from the clutter, approached and caught
(Madsen et al., 2005). Thus
our data strongly suggest that Ziphius and Mesoplodon
perform such deep and long dives to gain access to deep prey, as is the case
for the larger and better studied sperm whale. However, whereas sperm whales
typically perform consecutive deep dives with short intervening surface
intervals for ventilation (Watwood et al.,
2006), the tagged beaked whales almost always spent a protracted
time between deep dives, often filling this with a sequence of shallower,
apparently non-foraging dives. Our objective in the following sections is to
explore what this stereotypical behaviour reveals about the diving physiology
and potential vulnerability to decompression problems of Ziphius and
Mesoplodon.

Diving behaviour and the aerobic dive limit

In order to maximize foraging time in a deep prey patch, breath-hold divers
should minimise the time spent travelling to the surface, breathing, and
returning from the surface (Carbone and
Houston, 1996; Houston and
Carbone, 1992). Increased body size improves the potential for
breath-holding since larger animals can store more oxygen, and have lower
mass-specific diving metabolic rates
(Castellini et al., 1992;
Kooyman et al., 1983;
Schreer and Kovacs, 1997).
Mori (Mori, 2002) predicted
that, to forage optimally, smaller divers would rely more heavily on anaerobic
metabolism and would push closer to their physiological limits than larger
divers. This may require a refractory period between deep dives to clear the
lactic acid accumulated from anaerobic catabolism. The aerobic dive limit
(ADL) defines how long an animal can dive using aerobic respiration without
accumulating significant amounts of lactic acid. The ADL for Weddell seals,
one of the few species for which this has been measured, is about 21 min for
an adult of 450 kg (Kooyman et al.,
1980) and almost all of the dives (>90%) that these seals
undertake are shorter than the measured ADL. The average foraging dive time
for the larger elephant seal is less than 30 min
(Hindell et al., 1991;
Le Boeuf et al., 1988). In
comparison, the mean dive time measured by DTAGs during deep dives of
Mesoplodon is about 47 (s.d. 8) min and, for Ziphius, about
58 min (s.d. 11). Considering the high oxygen storage capacity and extreme
hypometabolism reported for both of these seal species
(Kooyman, 1989), it is
relevant to evaluate whether the beaked whales perform such long dives within
their ADL.

Diving metabolic rates have never been measured in beaked whales and these
species are likely to have specialized metabolic adaptations for deep-diving
as found for deep-diving seals. In the absence of other data, we propose a
first order approximation for the ADL of beaked whales estimated by
extrapolating the mass-specific diving metabolic rate of Weddell seals
(Castellini et al., 1992). The
large oxygen carrying capacity of up to 93 ml O2 kg-1 in
Weddell seals (Davis and Kanatous,
1999) is among the highest measured in any diving animal
(Kooyman, 1989). The
mass-specific basal metabolic rate of mammals generally scales by the power of
-0.25 (Kleiber, 1975), meaning
that the metabolic rate per unit of body mass decreases with increasing size.
Assuming that beaked whales can carry as much oxygen per kg body mass as the
extreme Weddell seal (Kooyman,
1989), and that the diving metabolic rate of the two species
scales with lean body mass (Mb) by the same power as the
basal metabolic rate of mammals in general, the ADL of a beaked whale can be
estimated by:
(2)

Using lean body masses of 630 kg and 2000 kg for adult Mesoplodon
and Ziphius, respectively [21% adipose tissue assumed for both
species (Mead, 1989)], the
predicted mass-specific diving metabolic rates are 85% (Md) and 64% (Zc) of
the rate of an adult Weddell seal with a lean body mass of 330 kg [27% adipose
tissue (Davis and Kanatous,
1999)]. These reduced diving metabolic rates as a result of larger
size would translate to aerobic dive limits of 25 min for a
Mesoplodon and 33 min for a Ziphius compared to 21 min for
the smaller Weddell seal (Kooyman et al.,
1980) assuming the same overall activity level during foraging
dives. Although such extrapolations must be treated with caution, the observed
mean foraging dive times for Mesoplodon and Ziphius are
about twice as long as their estimated ADLs, leaving a considerable margin for
error. It seems unlikely that the beaked whale species studied here would be
able to carry significantly more oxygen per kg body mass than the already
extreme amounts of the Weddell seal, or would deviate sufficiently from the
scaling of mass-specific diving metabolic rate to account for such a large
discrepancy. Consequently, we infer that most, if not all, foraging dives
presented in this paper likely have durations well in excess of the ADL and
that the animals return to the surface with an oxygen debt. In comparison,
applying the ADL scaling from a Weddell seal to the lean mass for sperm whales
estimated to weigh between 8 and 20 tonnes, the estimated ADL would range from
43-54 min, which is close to the mean dive duration of 45 (s.d. 6.3) min
reported for sperm whales of this size class
(Watwood et al., 2006). A 12
tonne sperm whale is 15 times heavier than Mesoplodon and six times
heavier than Ziphius, yet the average foraging dive duration of
Mesoplodon is comparable, and that of Ziphius, is
considerably longer. Whereas sperm whales dive near their ADL, the two beaked
whale species appear to exceed their ADLs by a factor of about two. Sperm
whales can perform deep dives with inter-deep-dive surface intervals averaging
only 9 min (Watwood et al.,
2006), but the beaked whales have intervals averaging 63 (Zc) and
92 (Md) min between deep foraging dives. Our explanation for this difference
is that the smaller species require protracted periods between deep dives to
process lactic acid accumulated during dives, which are partially supported by
anaerobic metabolism.

The question that arises is why the beaked whales studied here perform
foraging dives that so greatly exceed their ADL when this appears to
necessitate prolonged periods between foraging dives and therefore a reduction
in the time available for foraging at depth. The depth range of buzzes
(Johnson et al., 2004;
Madsen et al., 2005) and the
stomach contents of stranded individuals
(Blanco and Raga, 2000;
Mead, 1989;
Podesta and Meotti, 1991;
Ross, 1984;
Santos et al., 2001) both show
that these species have specialized to a deep prey niche with mean buzz depths
of round 724 m for Md and 863 m for Zc
(Fig. 1,
Table 2). The transport time to
and from this deep foraging zone represents a large fixed cost for each
foraging dive. Applying the mean ascent and descent rates measured for the two
species in this study, the transport time to and from foraging layers at 724 m
(Md) and 863 m (Zc) would be 25 min for Mesoplodon and 30 min for
Ziphius. Consequently, Mesoplodon would not be able to spend
any time at its foraging depth if it were to stay within its estimated ADL,
and Ziphius would have 3 min available for foraging. This remarkable
outcome is due in part to the slow ascent rate adopted by all of the tagged
beaked whales: the average ascent rate was less than 50% of the descent rate
for both species. This contrasts with sperm whales which were found to ascend
as fast or faster than they descended
(Watwood et al., 2006). It
therefore seems that these beaked whales must dive well beyond their ADL in
order to feed at depth, even though this incurs the cost of a longer time to
recover from the oxygen debt. Given an average deep dive cycle of 121 min for
Ziphius and 139 min for Mesoplodon, these species probably
can perform about 10-12 dives per day, each one enabling about 28 (Zc) or 22
(Md) min at foraging depth, for a total of between 4 and 5.6 h of foraging per
day. In comparison, the sperm whale with its short IDDI can perform more than
20 dives per day achieving a foraging time in excess of 9.5 h per day [based
on 28.5 min foraging per dive, after Watwood et al.
(Watwood et al., 2006)].

The apparent reliance on a combination of aerobic and anaerobic metabolism
during every foraging dive in beaked whales differs from most marine mammals
(Kooyman and Ponganis, 1997),
but is similar to some penguins (Ponganis
et al., 1997), sea lions
(Costa et al., 2001;
Chilvers et al., 2006) and fur
seals (Mattlin et al., 1998),
which seem to exceed their ADL regularly during foraging dives. Kooyman and
Ponganis (Kooyman and Ponganis,
1997) suggest that the small body size of these animals forces
them to exceed their ADL in order to access food sources at depths of more
than 300 m: they are obliged to make do with a less efficient diving behaviour
out of the necessity to reach deep prey to which there is a long two-way
travel time (Costa et al.,
2001). We propose that the beaked whales studied here, which are
relatively small compared to sperm whales, follow the same pattern and that
the long periods of acoustically-inactive shallow diving between foraging
dives therefore represent a period of recovery from the build-up of
metabolites of anaerobic metabolism. Castellini et al.
(Castellini et al., 1988)
measured a decline in lactate during short shallow dives performed by Weddell
seals after long dives in excess of the ADL and argued that this kind of dive
serves a recovery function. In terrestrial mammals, moderate exercise after
anaerobic lactate build up, termed active recovery, enhances lactate clearance
by increased perfusion of the lactate-producing tissues and by increased
oxidative metabolism (Bangsbo et al.,
1996). The fluking record during the beaked whale shallow dives
shows a mixture of gliding in the descent and steady fluking in the ascent
with active swimming during about 50% of the dive duration. This exercise
could improve foraging efficiency by speeding up recovery from the oxygen
debt. Thus, both physiological considerations and the characteristic diving
behaviour with long IDDIs are consistent with the hypothesis that these
comparatively small animals exceed their ADL to forage deeper on average than
any other marine mammal studied so far.

It is worth noting here that Ziphius and Mesoplodon are
so difficult for humans to observe because of their brief surface time. If
these species are at risk of predation from predators that spend most of their
time near the surface or that use vision, then the shallow dives may serve for
predator avoidance as well as recovery, perhaps explaining the `shallow' dives
of up to 425 m depth performed by the Ziphius. The most likely
predator of these species is the killer whale, Orcinus orca, which
spends >70% of its time shallower than 20 m
(Baird et al., 1998). Mead
(Mead, 1989) reported rake
marks on Mesoplodon produced by Orcinus or
Pseudorca, and Notarbartolo-di-Sciara
(Notarbartolo-di-Sciara, 1987)
reported a killer whale feeding on a fresh Ziphius carcass near our
Ligurian Sea study site, suggesting predation. Le Boeuf and Crocker
(Le Boeuf and Crocker, 1996)
consider deep-diving in elephant seals as an adaptation to reduce encounters
with near-surface predators such as killer whales and great white sharks.
Killer whales and other large mammal-eating odontocetes have sensitive hearing
and the fact that tagged beaked whales only produced their directional
high-frequency echolocation clicks at depths >200 m may be viewed as an
adaptation to avoid acoustic detection by a predator that seldom dives so
deep.

Implications for supersaturation and embolism

Jepson et al. (Jepson et al.,
2003)‡
and Fernandez et al. (Fernandez et al.,
2005) have reported gas and fat emboli in deep-diving beaked
whales that stranded soon after naval sonar exercises. They conjectured that
bubble formation in supersaturated tissues may have been caused by
acoustically driven bubble growth or by a disruption of normal diving
behaviour such as accelerated ascent rates. The process of decompression is
quite different for breath-hold divers than for divers who breathe compressed
gases. During descent, as the ambient pressure increases, the lungs of
breath-hold divers collapse as the fixed amount of air diffuses from alveoli
to blood or moves into the more rigid and thick-walled trachea and nasal
apparatus (Scholander, 1940).
Lung collapse is estimated to occur at 25-60 m for seals that exhale before
diving (Falke et al., 1985;
Kooyman et al., 1972) and at
70 m in a bottlenose dolphin that dives on inhalation to have air available
for sound production at depth (Ridgway and
Howard, 1979; Ridgway et al.,
1969). The assumption that beaked whales also dive upon inhalation
is consistent with the observation that the fluke rate decreased after the
first 20-40 m of descents by both species without loss of speed, presumably
due to a reduction in buoyancy as the lungs compressed. The depth of lung
collapse is not known for beaked whales, but if it is assumed, as with
dolphins, that the comparatively rigid volume of the bronchi, trachea, bony
nasal passages, and air spaces of the nasal complex represent more than 9% of
the total surface air volume in beaked whales, there should be no more air in
the alveoli at a depth of 100 m
(Scholander,
1940)¶.

According to Fick's law, the diffusion rate of a gas per unit time across a
tissue barrier is a product of the difference in partial pressures, the
diffusion area, and the inverse of the barrier thickness
(Dejours, 1975). The partial
pressure gradient of nitrogen between the airspaces and the blood increases
with increasing hydrostatic pressure, promoting diffusion of gas. However,
when air moves out of the alveoli (which have a large perfused surface area
and a thin one cell layer between air and blood) and into the airspaces of the
trachea and the head, the drastically reduced surface areas and much increased
barrier thickness will result in a very slow diffusion rate
(Crystal et al., 1997). Thus,
when marine mammals dive deeper than the depth of alveolar collapse, there is
no effective interface left for gas diffusion into the blood
(Falke et al., 1985;
Irving, 1935;
Kooyman et al., 1972;
Scholander, 1940) and the
nitrogen tensions in the blood and muscle stabilize or drop despite increasing
hydrostatic pressure, as has been observed in diving seals
(Falke et al., 1985;
Kooyman et al., 1972) and
dolphins (Ridgway and Howard,
1979). Accordingly, no significant amount of nitrogen can be
expected to diffuse from airspaces to the blood during the portions of dives
that are deeper than the depth of lung collapse.

Nonetheless, observations consistent with DCS and embolism have been made
in stranded beaked whales of the genera Ziphius and
Mesoplodon (Jepson et al.,
2003; Jepson et al.,
2005; Fernandez et al.,
2005) and, as shown in this paper, these animals employ an extreme
diving behaviour with a stereotypical form unusual in diving air-breathers. In
the following we examine three peculiar aspects of this diving behaviour for
indications that these species of beaked whales have a heightened risk of DCS.
The three behaviours are: (1) the occasional protracted periods near the
surface, (2) the sequences of shallow dives following deep dives, and (3) the
slow ascents from deep dives.

A breath-holding animal, submerged at a depth shallower than that of lung
collapse, will have a nitrogen influx to the blood, due to the high partial
pressures of the compressed air in the lungs. If submergence is maintained for
long enough, the blood will be supersaturated with nitrogen (i.e.
N2 tissue tension > atmospheric partial pressure of
N2) upon returning to the surface
(Brubakk and Neuman, 2003;
Falke et al., 1985;
Kooyman et al., 1972).
However, the high rate of lung perfusion and low carrying capacity of nitrogen
in the blood will provide rapid equilibration of the nitrogen tensions at the
surface (Cross, 1965;
Falke et al., 1985;
Fahlman et al., 2006) unless
shallow dives are repeated over and over with short surface intervals
(Paulev, 1965;
Paulev, 1967;
Ridgway and Howard, 1979) to
supersaturate the tissues such as fat and muscles of higher carrying capacity.
Cox et al. (Cox et al., 2006)
speculated that the characteristic short surface intervals of beaked whales
may indicate that they are chronically supersaturated with nitrogen to a level
at which remaining near the surface for more than a few minutes could be a
risk factor for emboli. Our observations of beaked whales spending up to 86
min (Zc) and 33 min (Md) at mean depths of less than 11 m from the surface
(including prolonged logging directly at the surface) after deep foraging
dives do not support this contention. Rather it seems that these whales, like
other diving animals, equilibrate to normal nitrogen tensions during surface
intervals.

Despite the lack of evidence for dangerous levels of chronic
supersaturation, it is tempting to regard the characteristic sequence of
increasingly shallow dives following deep foraging dives
(Fig. 4) as recompression
dives, i.e. as serving to prevent emboli by slowly equilibrating the gas
tensions in the tissues, analogous to the recompression process adopted by
human divers (Brubakk and Neuman,
2003; Paulev,
1967). If shallow dives function for decompression, one would
hypothesize that the animals would descend rapidly to a depth close to lung
collapse where the pressure difference between the supersaturated tissue and
the compressed air in the alveoli is smaller, and then slowly return to the
surface, allowing gas to diffuse back from the blood and the tissues with
little risk of bubble formation (Brubakk
and Neuman, 2003). Our finding that the descent phase of shallow
dives takes, on average, a little longer than the ascent, is contrary to this
prediction and suggests that shallow dives if anything would facilitate
supersaturation, not prevent it. In addition, many of the short dives
(especially of Ziphius) are considerably deeper than the 100 m
expected maximum depth of lung collapse. There is accordingly little support
for the hypothesis that the shallow dives after deep, foraging dives serve as
recompression dives at least within the mechanisms known for dolphins
(Ridgway and Howard, 1979;
Ridgway et al., 1969), seals
(Falke et al., 1985;
Kooyman et al., 1972) and
humans (Brubakk and Neuman,
2003; Paulev,
1967). On the contrary, the shallow dives of beaked whales would
seem to pose a greater risk for the generation of supersaturated tissue and
subsequent decompression stress than the deep, long foraging dives, where the
alveoli are collapsed for most of the dive. The highest nitrogen tensions in
the muscles of a dolphin were measured after a long series of dives to 100 m,
whereas deeper dives yielded lower nitrogen tensions
(Ridgway and Howard,
1979).

The third enigmatic aspect of beaked whale dive behaviour that may have a
bearing on the risk of DCS is the consistent slow ascent from deep dives.
Given that these animals most likely exceed their ADL during deep foraging
dives and must spend protracted times at the surface between such dives to
recover, it is puzzling that the ascent rate in deep dives is, on average, one
half that of the descent rate. To maximize time spent foraging, one would
expect either a rapid return to the surface at the end of foraging, or that
foraging would continue during a slower ascent. However, all tagged whales,
without exception, made slow silent ascents from deep foraging dives
suggesting that this behaviour is either physiologically mandated or
behaviourally important. In the following, we consider a number of hypotheses
for slow ascents culled from the literature, and discuss why none of these
seem to fit the behaviour observed in the tagged beaked whales.

One possible explanation for the slow ascent rate is that a whale, already
at or near its ADL by the end of the foraging phase of a deep dive, might seek
to minimize the cost of transport by performing a long duration, but
metabolically cheap, gliding ascent as described for Weddell seals
(Sato et al., 2003) and sperm
whales (Miller et al., 2004).
However, tagged beaked whales swam with consistent fluking effort at a shallow
pitch angle during the long ascents (Fig.
3), resulting in a swimming speed probably comparable to that of
the descent, but with a smaller vertical component.

Sato et al. (Sato et al.,
2004) reported that foraging penguins sometimes extend their
ascent, returning to the surface at a low pitch angle, and suggested that this
behaviour may serve to increase horizontal displacement from a poor food
patch. Tagged beaked whales always ascended at a low pitch angle and
consistently covered more than 50% of the possible horizontal displacement
during ascent. No correlation was found between horizontal displacement and
buzz count with the small data set available, suggesting that the ascent
behaviour is decoupled from variation in food patch quality and that the
whales employ this behaviour for other reasons. It is possible that the
consistently high horizontal displacements achieved during the silent ascents
could impede a near-surface predator from tracking these cryptic species after
they stop vocalizing at depth.

Slow ascents from deep dives have also been reported for another beaked
whale species, the northern bottlenose whale, Hyperoodon ampullatus
(Hooker and Baird, 1999), and
for beluga whales, Delphinapterus leucas
(Martin and Smith, 1992). Both
of these species reduced their vertical velocity in the top 200 m of ascents
from deep dives. The authors noted that the changes in gas volume with depth
are greatest near the surface and suggested that the whales they studied might
slow the final phase of their ascents to reduce the risk of gas bubbles
forming in the blood or tissues, or to avoid fast expansion of gas in confined
and possibly constricted airspaces such as sinuses. Enigmatically, the beaked
whales studied here ascended slowly at the start or in the middle of their
ascent, depending on dive depth, where changes in gas volume with depth are
small. In the top 100 m, where gas expansion is most rapid, ascent rates were
variable but often comparable to descent rates. This pattern is inconsistent
with the hypothesis that slow ascents serve to reduce the negative effects of
gas expansion in the lungs or other tissues.

Jepson et al. suggest that a rapid ascent from a deep dive may heighten the
risk of nitrogen supersaturation and subsequent bubble formation
(Jepson et al., 2003). Under
this argument, the slow ascents could provide time for gas to diffuse back
into the airspaces and so avoid embolism. For this to be the case, there must
be a positive gradient from the blood to the gas phases and an interface over
which diffusion can happen effectively within the duration of the slow part of
the ascent. Given that Ziphius ascend most slowly between 600 and 400
m, spending an average of 6 min in this depth range, the whale would need some
tissues saturated with nitrogen at tensions greater than 35 000 mmHg (61 ATA×
580 mmHg of N2 at the surface) at the end of the foraging
phase to have a positive diffusion gradient from the blood to the air spaces
at 600 m depth. Considering the time available for diffusion, the interface
and barrier thickness problems and the half time of nitrogen exchange in
tissues (Ridgway and Howard,
1979), we regard such high N2 tensions as
inconceivable. On the contrary, we argue that, if beaked whales actually do
have significant nitrogen flux between airspaces and the blood at great
depths, ascending slowly from 600 to 400 m will only facilitate higher tissue
tensions of nitrogen, not prevent them. In this case, it would be better to
move quickly up to the depth of lung collapse where there is an effective
interface and larger partial pressure gradient in the re-inflating alveoli,
and then ascend slowly from there to the surface, as described for
Hyperoodon and beluga. We did not observe such behaviour and conclude
that the slow ascent at depth and the faster, but variable ascent in the upper
100 m is inconsistent with mediating the effects of nitrogen supersaturation.
The slow ascent is costly and must serve a critical behavioural or
physiological purpose, but, given current models of breath-hold diving,
mitigating decompression problems cannot in our view explain this unusual
behaviour. The simple model for nitrogen diffusion considered here does not
take into account the role of other tissues in the body in absorbing and
releasing nitrogen, since we have no information about perfusion rates of such
tissues in beaked whales during diving. However, given the current lack of
evidence for severe supersaturation in the blood, it seems unlikely that
nitrogen stored in other compartments would radically change the risk of
emboli under normal diving conditions.

Several aspects of the diving behaviour of beaked whales remain enigmatic:
we cannot offer an explanation for the slow ascents observed from deep dives
nor for the sequences of shallow dives with decreasing depth following a deep
dive. The diving behaviour of beaked whales must fit within physiological
constraints but also must meet ecological and behavioural needs related to
foraging, social interactions and predator avoidance. Although our analysis of
the normal dive behaviour of beaked whales does not identify an obvious risk
factor for decompression problems, the enigmatic elements of diving behaviour
and the apparent vulnerability of beaked whales to anthropogenic sound may not
correspond exclusively to physiological limitations, but could also be related
to behavioural issues, such as problems maintaining social cohesion given the
limited range of their acoustic signals, or the strategies they use to avoid
predation.

Scenario for development of decompression sickness in beaked
whales

Even though beaked whales are extreme divers, our analysis above does not
suggest that these animals run a risk of decompression stress and embolism
during normal diving behaviour. However, an aberrant behaviour can be
envisioned, for which decompression sickness might be an issue. Houser et al.
(Houser et al., 2001) modelled
the nitrogen levels in the tissues of a diving bottlenose whale based on a set
of assumptions including lung collapse at a depth of 70 m. They concluded that
diving speed and depth are the primary determinants of tissue nitrogen
accumulation, with slower rates of ascent/descent and dives within lung
collapse depths leading to supersaturation. Thus, considerable time spent
close to, but within, lung collapse depth should cause the highest risk for
supersaturation and thereby bubble formation. The highest influx of nitrogen
does not happen just before the depth of complete alveolar collapse, because
the process of alveolar collapse is gradual. So while the partial pressure
gradient between nitrogen in the lungs and the blood perfusing the alveoli
increases with depth, the surface area over which the diffusion takes place
decreases with the reducing lung volume. The so-called invasion rate of
nitrogen (Scholander, 1940) is
therefore estimated to be highest at depths about half-way between the surface
and the depth of complete alveolar collapse. Beaked whales with a lung
collapse depth shallower than 100 m will therefore probably experience the
highest influxes of nitrogen when they are at depths between 30 and 80 m.

This line of reasoning suggests that the most risky behaviour in terms of
decompression sickness for beaked whales would be repeated dives at depths
between 30 and 80 m. If a whale responded to a noise source by repeating
several long dives in this depth range with short intervening surface
durations, such a response could facilitate tissue supersaturation levels of
up to 400 to 900% (corresponding to 5-10 ATA) compared to the partial
pressures of nitrogen at the surface. Supersaturation at that level could lead
to embolism and symptoms of DCS (Brubakk
and Neuman, 2003) when the animals return to the surface (sensu
Paulev, 1965;
Paulev, 1967) for long periods
of time, e.g. because of fatigue or stranding. Thus if exposure to sonar pings
provokes a response of the sort described here, it could lead to DCS-like
symptoms in line with the pathological findings in the stranded animals, but
it does not explain why beaked whales seem at higher risk than the other
species. The critical question in this context is how these beaked whale
species respond to sounds of sonars and whether these responses pose a
physiological risk that can ultimately cause the animals to strand and
die.

Conclusion

The two species of beaked whales studied here, Ziphius cavirostris
and Mesoplodon densirostris, undertake long, deep dives to predate on
a deep water food source. Diving is highly stereotypical with most deep
foraging dives being followed by an extended period of shallow dives and slow
travel/resting near the surface. All foraging dives of both species are
considerably longer than the estimated aerobic dive limits, suggesting that
the whales return to the surface with an oxygen debt. We propose that the
shallow dives and the long periods in-between foraging dives are needed to
repay the oxygen debt before the next deep dive. Despite the extreme diving of
these species, the long periods spent at or close to the surface are
inconsistent with the hypothesis that beaked whales are chronically
supersaturated at high levels. Likewise, the equal durations of ascent and
descent found in the shallow recovery dives are contrary to what would be
required for recompression. Another consistent feature of the dive profiles,
the slow ascent from deep foraging dives, remains a puzzle. The long ascents,
which are acoustically inactive but involve active swimming, appear to divert
substantial time away from foraging, suggesting that the animals are
constrained by some physiological requirement or behavioural need that
prevents them from optimizing foraging performance. However, since the slowest
phase of the ascent occurs well below the depth of lung collapse, we argue
against it representing an adaptation to prevent bubble formation. Rather we
argue that the deep-diving behaviour of beaked whales is unlikely, under
current models of nitrogen diffusion, to heighten the risk of embolism. We
suggest that emboli observed in animals exposed to naval sonar exercises
(Jepson et al., 2003;
Jepson et al., 2005;
Fernandez et al., 2005) could
stem instead from a behavioural response that involves repeated dives
shallower than the depth of lung collapse. Regardless of the precise cause of
the strandings, it is a pressing issue to develop effective mitigation
protocols so as to reduce the accidental exposure of beaked whales to navy
sonar. The stereotyped dive cycle and acoustic behaviour of Ziphius
and Mesoplodon described in this paper provide critical data for the
design of efficient acoustic and visual detection methods for the at-risk
species.

ACKNOWLEDGEMENTS

We would like to thank for fieldwork assistance: D. Allen, C. Aparicio, M.
Ballardini, A. Bocconcelli, F. Borsani, A. Brito, F. Díaz, I.
Domínguez, M. Grund, M. Guerra, F. Gutierrez, A. Hernandez, C.
Militello, G. Mojoli, B. Nani, M. Podesta, T. Pusser, G. Ramella, E. Revelli.
T. Pusser also provided critical references concerning killer whale predation
on beaked whales. We also thank D. Allen, D. Costa, M. Fedak, G. Kooyman, G.
Lykkeboe, H. Malte, J. Mead, P. Paulev, S. Ridgway, T. Wang and an anonymous
reviewer for helpful discussions and critique. Fieldwork support was provided
by BluWest, NATO Undersea Research Center, and the Government of El Hierro.
Funding for tag development was provided by a Cecil H. and Ida M. Green Award
and the US Office of Naval Research. Funding for field work was provided by
the Strategic Environmental Research and Development Program (SERDP) under
program CS-1188, the National Ocean Partnership Program, the Packard
Foundation, the Canary Islands Government (D.G. Environment) and the Spanish
Ministry of Defense. P.T.M. is currently funded by a Steno Fellowship from the
National Danish Science Foundation. Research was conducted under US NMFS
permits 981-1578-02 and 981-1707-00 and a permit from the government of the
Canary Islands. This research was approved by the Woods Hole Oceanographic
Institution Institutional Animal Care and Use Committee.

↵‡ Gas-filled cavities were also reported for delphinids stranded in Britain,
with no known concurrent sonar activity.

↵¶ If the relative volume of the upper airways is the same for beaked whales
as for dolphins, the above estimate is conservative since the relative lung
volume of a beaked whale is estimated to be about half that of a dolphin
(Scholander, 1940), so it is
likely that the depth of lung collapse is shallower than 100 m and possibly 70
m in beaked whales. This estimate may also be biased deep because it ignores
gas that diffuses from the alveoli into circulation.

Bennett, P. B. (1982). The high pressure
nervous syndrome in man. In The Physiology and Medicine of Diving
and Compressed Air Work (ed. P. B. Bennett and D. H. Elliot), pp.262
-296. London: Balliere-Tindall.